Snap switches are frequently used in electrical engineering in order to assure that switching over of electrical contacts take place abruptly, as soon as an arbitrarily slow variable input variable has reached a certain critical value. Abrupt switchings over are necessary in order to avoid contact burning and to achieve sure contact. Frequently, for example in the case of membrane pressure switches, the input variable is a force which must overcome an adjustable prestress in order to bring about switching over of the snap switch.
With a known snap switch of the described type, the prestress spring is a helical pressure spring which is supported on the one hand on an adjusting screw and on the other hand presses directly against the guide rod. The guide rod is mounted on a physically formed axis, which is fixed in such a way that the guide rod solely transmits switching forces to the snap spring, but relieves this of transverse forces. This applies for the transverse force components of the actuating force acting on the guide rod as well as for the transverse force components which can be transmitted from the prestress spring to the guide rod. A non-rotatable intermediate member is arranged between the helical spring and the adjusting screw, which intermediate member should prevent turning the helical spring along when the adjusting screw is turned to adjust the snap switch. Thus, the helical spring is prevented from building up a torsion stress, which torsion stress would be first maintained by static friction, but would sooner or later be released due to vibrations of the snap switch, due to which the prestress acting on the guide rod, and thus the entire switching characteristic of the snap switch would change by an amount which is not predetermined.
Despite these measures--on the one hand relieving the snap spring of transverse forces, which could change the snapping over behavior, and on the other hand efforts to keep the prestress force constant--it is difficult with known snap springs of the described type and, it is possible, if at all, only with great manufacturing-technical effort, to guarantee over the long run that a switching over process takes place when and only when the input variable reaches or exceeds a critical value predetermined with narrow tolerances. On the other hand, these difficulties are due to the fact that the position of the helical spring with respect to the guide rod is not sufficiently definable and due to this it cannot be ruled out that with adjusting of the known snap switch, certain stresses are maintained in the helical spring due to static friction only and are later released due to vibrations. Consequently, the position of the spring axis can change with relation to the rotational axis of the guide rod, which results in a corresponding change of the torque exerted by the helical pressure spring on the guide rod. On the other hand, the magnitude of bearing friction, which resists the swivelling of the guide rod, is dependent with the described known snap switches on manufacturing tolerances, accidental shifts of the guide rod along its axis as well as bearing corrosion possibly occurring in the course of time, and therefore cannot be taken into account in advance over a long period of time with the adjustment of the snap switch.